This is a place of arrivals and departures, a centuries-old harbour where few stop for long. From hosting the steamships and trans-Atlantic liners of the past to today’s colossal cross-channel ferries, Plymouth’s historic Millbay dock has served the comings and goings of countless travellers.

On work lunch breaks I regularly walk beside the innermost basin, now transformed into a marina as part of the area’s regeneration. Solid harbour walls designed by Isambard Kingdom Brunel provide shelter for luxury yachts neatly berthed along wooden pontoons.

King Point marina at Millbay dock, Plymouth. Photograph: Charlie Elder

Apart from gulls and the occasional cormorant, there is seldom much to see in the way of wildlife.

It was a thin trail of bubbles that caught my eye, sparkling at the surface in the quieter eastern corner. I doubled back for a closer look, wondering whether it had been caused by a shoal of fish. The tide was high and the fine beads of air rose just a few feet below me, following the line of the stone dockside, but despite the noonday sunshine the source remained hidden in the depths.

A pause, a few larger bubbles, then the trail led to a neighbouring floating pontoon and I heard something surface beneath the boards, out of view of nearby yacht owners.

Only on the third dive did the creature reveal itself: a dark streamlined body tapering into a strong tail, bubbles streaming from nose and fur, back legs pumping as it moved through a shaft of light and back into the shadows. An otter.

Devon is a stronghold for otters and the harbour lies near a river estuary, but the fact that this wily adult was actively hunting in the middle of the day at such a busy coastal location was still remarkable.

In the days since I have not had a second sighting, but the eastern corner of the harbour will keep drawing me back for another look, in the hope that this secretive visitor once again returns to this place of brief encounters.